The present invention relates to optical waveguide devices and, in particular, to optical waveguides tunable by electrowetting actuation of fluids in proximity to those devices.
Optical fibers are useful for many applications in modern communications systems. A typical optical communications system comprises a transmitter of optical signals (e.g., a laser-based transmitter that generates a desirable wavelength of light, such as 1550 nm), a length of transmission optical fiber coupled to the source, and a receiver coupled to the fiber for receiving the signals. Optical fiber useful in such systems typically comprises a strand of wave-guiding glass with an inner core region and an outer cladding region that surrounds the core. As long as the refractive index of the core exceeds that of the cladding, a light beam can be guided along the length of the core by total internal reflection. One or more amplifying systems may be disposed along the fiber for amplifying the transmitted signal.
Filters and attenuators, such as those comprising an optical fiber grating, are required in these systems to change the power levels and characteristics of various signals or portions of signals propagating through an optical fiber. An optical fiber grating typically comprises a length of fiber including a plurality of optical grating elements such as index of refraction perturbations, slits or grooves. These elements may or may not be substantially equally spaced. Illustrative examples of such gratings include Bragg gratings and long-period gratings.
A fiber Bragg grating comprises a length of optical fiber including a plurality of perturbations in the index of refraction. These perturbations selectively reflect light of wavelength xcex equal to twice the distance xcex9xe2x80x2 between successive perturbations times the effective refractive index, i.e.:
xcexR=2neff-corexcex9xe2x80x83xe2x80x83(Equation 1)
where xcexR is the vacuum wavelength and neff-core is the effective refractive index of the propagating mode. The remaining wavelengths pass through the grating essentially unimpeded. Bragg gratings have found use in a variety of applications including filtering, adding and dropping signal channels, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and compensation for waveguide dispersion.
A long period grating couples optical power between two copropagating modes with very low back reflections. It typically comprises a length of optical waveguide wherein the refractive index perturbations are spaced by a periodic distance xcex9 that is large compared to the wavelength xcex of the transmitted light. In contrast with Bragg gratings, long-period gratings use a periodic distance xcex9 which is typically at least 10 times larger than the transmitted wavelength, i.e., xcex9xe2x89xa710xcex. Typically xcex9 is in the range of 15xcx9c1500 micrometers, and the width of a perturbation is in the range of ⅕ xcex9 to ⅘ xcex9. In some applications, such as chirped gratings, the spacing xcex9 can vary along the length of the grating. Long-period gratings are particularly useful in optical communication systems for equalizing amplifier gain at different wavelengths. See, for example, U.S. Pat. No. 5,430,817 issued to A. M. Vengsarkar on Jul. 4, 1995.
Many potential applications require optical gratings wherein light propagation behavior through the grating is tunable. A tunable long period grating, for example, can provide dynamic gain compensation. On the other hand, a tunable Bragg grating can permit dynamic control over which wavelength will pass through the grating and which will be reflected or diverted. While this tunability is desired, light is confined mostly in the core region of an optical fiber and, therefore, the ability to externally effect propagation behavior of the light is significantly limited. With conventional fibers, one is essentially limited to the application of strain and/or temperature changes to the fiber to change the propagation behavior of light in the core. Alternatively, specially designed microstructured fibers have been developed whereby small quantities of fluid are pumped into channels disposed within the structure of the fiber itself. Such fibers are the subject of copending U.S. patent application Ser. No. 10/094,093, entitled xe2x80x9cTunable Microfluidic Optical Fiber Devices And Systems,xe2x80x9d which is hereby incorporated by reference herein. While prior tunable optical devices are acceptable for many uses, they tend to be limited in their effect on light propagation behavior and can be expensive to manufacture.
As optical communications systems become more advanced, there is a growing need for new, cost-effective tunable optical devices and methods of using those devices for changing the propagation behavior of light signals through optical waveguides.
A tunable optical waveguide device is enclosed in an enclosure containing a region of fluid with a refractive index different than the optical waveguide. The region of fluid is controllably moved within the enclosure to modify at least a first transmission property of the device in the region to which or from which the fluid is moved in order to variably attenuate the wavelengths of the signal transmitted through the waveguide. In a first embodiment, the optical waveguide device comprises an optical fiber long-period grating that is tuned by moving the fluid over the grating to vary the amplitude of desired wavelengths that are transferred into the cladding of the fiber and, as a result, to decrease the amplitude of those desired wavelengths that are transmitted through the core of the fiber. In a second embodiment, the optical waveguide device comprises an optical fiber Bragg grating that is tuned by moving the fluid over the grating to vary the amplitude of desired wavelengths that are reflected back through the core of the fiber.